Solid electrolyte ion conductive transport membranes have significant potential for the separation of oxygen from oxygen-containing gas. Membranes that are of particular interest include mixed conductor materials that conduct both oxygen ions and electrons and which can operate in a pressure driven mode without the use of external electrodes. Certain mixed conductor membranes are known in the art. For example, composite ceramic mixed conductor membranes comprised of multiphase mixtures of an electronically-conductive material and an oxygen ion conductive material for electrochemical reactors and partial oxidation reactions have been disclosed by T. J. Mazanec et al. in U.S. Pat. No. 5,306,411. Composite mixed conductor materials containing oxygen ion-conducting materials such as bismuth oxide and electronically conductive materials have been disclosed by M. Liu et al. in U.S. Pat. No. 5,478,444. True mixed conductors, exemplified by perovskites such as La.sub.1-X Sr.sub.X CoO.sub.3-Y, La.sub.X Sr.sub.1-X FeO.sub.3-Y, La.sub.X Sr.sub.1-X Fe.sub.1-Y Co.sub.y O.sub.3-Z and others, are materials that possess intrinsic conductivity for both electrons and ions.
Oxygen transport is driven by the partial pressure of the oxygen in the gas streams. The Nernst potential is developed internally, and drives the flux of oxygen vacancies against the ionic resistance of the electrolyte as disclosed in commonly assigned U.S. Pat. No. 5,547,494 by Prasad et al. entitled "Staged Electrolyte Membrane", which is incorporated herein by reference.
Generally, thin electrolyte films are desirable because the ideal oxygen flux is inversely proportional to their thickness. Thus, thinner films lead to higher oxygen fluxes, reduced area, lower operating temperatures and lower oxygen pressure differentials across the electrolytes. Also, when the anode-side of the membrane is purged with a reactive gas, such as methane or hydrogen, the oxygen activity on the anode side reduces significantly, thus leading to higher oxygen flux across the membrane. However, as the oxygen flux increases, the surface resistance to transport become more pronounced and ultimately dominate the overall resistance to transport. Surface resistance arise from various mechanisms involved in converting an oxygen molecule in the gas phase to oxygen ions in the crystal lattice and vice versa. Even for dense ion transport membranes, surface kinetics on the cathode or the anode side may limit the oxygen flux across the membrane.
Yasutake Teraoka et al. reported solid state gas separation membranes formed by depositing a dense mixed conducting oxide layer onto a porous mixed conducting support in Jour. Ceram. Soc. Japan. International Ed., Vol.97, No.4, pp.458-462 and No.5, pp.523-529 (1989). The relatively thick porous mixed conducting support provides mechanical stability for the thin, relatively fragile dense mixed conducting layers. Considering the thickness of the dense layer, the authors therein expected the oxygen flux to increase by a factor of ten for the composite thin film membrane as compared to a dense mixed conducting disk. However, they obtained an increase in oxygen flux of less than a factor of two using the thin film membrane.
Terry J. Mazanec et al. described solid multicomponent membranes comprised of gas impervious, multi-phase mixtures of an electronically-conductive phase and an oxygen ion-conductive phase and/or gas impervious, single phase mixed metal oxides having a perovskite structure and having both electron-conductive and an oxygen ion-conductive properties in U.S. Pat. No. 5,306,411. Dual phase electronic-ionic conductive material (generally requires electronic second phase of more than 30 vol. % to form a continuous second phase to enable operation above the percolation limit) for membranes was discussed. Dual phase electronic-mixed conductive material (second phase of less than 30 vol. % is dispersed in the mixed conductor matrix to enhance mechanical and catalytic properties) is described in commonly assigned U.S. Ser. No. 08/444,354 entitled "Solid Electrolyte Membrane with Mechanically Enhanced Constituents," and is hereby incorporated by reference.
It has been reported that ceramic membranes comprised of a mixed conducting perovskite with a porous coating of metal, metal oxide or combinations thereof increases the kinetic rate of the feed side interfacial fluid exchange, the kinetic rate of the permeate side interfacial exchange, or both.
U.S. Pat. No. 4,791,079 discloses catalytic ceramic membranes consisting of two layers, an impervious mixed ion and electronic conducting ceramic layer and a porous catalyst-containing ion conducting ceramic layer. The preferred composition for the porous ceramic layer is zirconia stabilized with approximately 8 to 15 mole percent calcia, yttria, scandia, magnesia, and/or mixtures thereof.
U.S. Pat. No. 5,240,480 discloses multi-layer composite solid state membranes, comprising a multicomponent metallic oxide porous layer and dense layer, which are capable of separating oxygen from oxygen-containing gaseous mixtures at elevated temperature.
U.S. Pat. No. 5,569,633 discloses surface catalyzed multi-layer ceramic membranes consisting of a dense mixed conducting multicomponent metallic oxide layer, and a catalyzed metal or metal oxide coating on the feed (air) side to enhance the oxygen flux. Catalytic coating on both sides did not enhance the oxygen flux.